Add pull request template

[KardelRuveyda]

What

Adds .github/pull_request_template.md. Once merged, every new pull request opened against this repo will be pre-populated with a short, opinionated checklist.

Why

• Helps contributors structure their PRs so maintainers can review faster.
• Surfaces a few things that are easy to forget but painful to clean up afterwards: large binaries, secrets, README updates after a behavioural change.
• Quietly directs contributors to CODE_OF_CONDUCT.md and SECURITY.md at the moment they are most relevant.

What's in the template

• Summary — one or two sentences.
• Type of change — checkbox list (bug fix, feature, breaking, docs, refactor, CI/housekeeping, other).
• Related issues — Closes #… style links.
• How has this been tested? — encourages mentioning runs on example_data/.
• Checklist — Code of Conduct, single topic, README update, no binaries, no secrets, security path.
• Additional notes — escape hatch for screenshots / benchmarks.

Notes for the maintainer

• Docs/config only; no code touched.
• Every section is optional — the template tells contributors they can delete sections that don't apply, so it stays lightweight.
• This PR itself was opened before the template existed in main, so it does not show the new checklist (GitHub only applies templates that are already on the base branch).